1. Field of the Invention
The present invention relates to a polar modulation transmission apparatus and a wireless communication apparatus, and more specifically to a polar modulation transmission apparatus for maintaining a modulation precision and a distortion characteristic at a superb level even at a low output and realizing a superb power efficiency, even with a modulation system providing a wide dynamic range, and a wireless communication apparatus using the same.
2. Description of the Background Art
Conventionally, as a high frequency power amplifier for amplifying a modulation signal including an envelope varying component, a class A or class AB linear amplifier is used in order to amplify the envelope varying component linearly. Such a linear amplifier is superb in linearity, but constantly consumes power which accompanies a DC bias component and so has a lower power efficiency than, for example, a class C or class E nonlinear amplifier. When such a high frequency power amplifier is used in a mobile wireless communication apparatus having a battery as a power source, the battery life is short because the power consumption of the high frequency power amplifier is high. When such a high frequency power amplifier is used for a base station apparatus of a wireless system including a plurality of high power transmission apparatuses, the scale of the base station apparatus is enlarged and the amount of heat generation is increased.
In order to solve these problems, a transmission apparatus using a polar modulation system, which is operable at a high power efficiency, has been proposed. According to the polar modulation system, a baseband signal is separated into an amplitude signal and a phase signal, and then amplitude modulation and phase modulation are performed separately (see, for example, FIG. 10). In the example shown in FIG. 10, a high frequency phase-modulated signal processed by phase modulation based on a phase signal is multiplied by an amplitude signal using a multiplier to generate a high frequency transmission signal. A transmission apparatus using the polar modulation system allows a high frequency power amplifier acting as a multiplier to operate in a nonlinear mode (in a saturated mode) and therefore can improve the power efficiency. Technologies using this type of polar modulation system are disclosed in, for example, patent document 1 (Japanese Patent No. 3207153) and patent document 2 (Japanese Laid-Open Patent Publication No. 2001-156554).
FIG. 11 shows an example of functional blocks of a conventional transmission apparatus 500 using the polar modulation system. As shown in FIG. 11, the conventional transmission apparatus 500 includes an amplitude/phase separation section 501, a multiplier 502, an amplitude signal amplifier 503, a phase modulation section 504, a variable gain amplifier 505, a high frequency power amplifier 506, a digital/analog converter (hereinafter, referred to as a “D/A converter”) 507, and a D/A converter 508.
The amplitude/phase separation section 501 separates an input baseband signal S51, which is to be modulated, into an amplitude signal S52 and a phase signal S53. The amplitude signal S52 and a transmission power control signal S54 for controlling the magnitude of transmission power are input to the multiplier 502. The multiplier 502 multiplies the amplitude signal S52 by the transmission power control signal S54, and outputs the resultant signal as a transmission power-controlled amplitude signal S55. The amplitude signal S55 is converted into an analog signal S56 by the D/A converter 507, and is supplied to the high frequency power amplifier 506 performing a nonlinear operation (saturated operation) via the amplitude signal amplifier 503 as a supply voltage S57.
On the other hand, the phase signal S53 is input to the phase modulation section 504. The phase modulation section 504 performs phase modulation on a carrier signal based on the phase signal S53, and outputs the resultant signal as a high frequency phase-modulated signal S58. The high frequency phase-modulated signal S58 is input to the variable gain amplifier 505. The gain of the variable gain amplifier 505 is controlled by an analog signal S62 obtained by D/A-converting a gain controlling digital signal S61 supplied from a control section (not shown). The variable gain amplifier 505 amplifies the high frequency phase-modulated signal S58 and outputs the resultant signal as a high frequency phase-modulated signal S59. The high frequency phase-modulated signal S59 is input to the high frequency power amplifier 506. The high frequency power amplifier 506 performs amplitude modulation on the high frequency phase-modulated signal S59 based on the supply voltage S57 supplied from the amplitude signal amplifier 503, and outputs the resultant signal as a high frequency transmission signal S60.
Next, an operation of the conventional transmission apparatus 500 will be described in detail using expressions. Where the baseband signal S1 to be modulated is Si(t), Si(t) is represented by expression (1) using complex numbers.Si(t)=a(t)·exp[jφ(t)]  (1)
Here, a(t) represents amplitude data, and exp[jφ(t)] represents phase data. The amplitude/phase separation section 501 extracts amplitude data a(t) and phase data exp[jφ(t)] from the baseband signal Si(t). The amplitude data a(t) corresponds to the amplitude signal S52, and the phase data exp[jφt)] corresponds to the phase signal S53. The amplitude data a(t) is multiplied by power information G, represented by the transmission power control signal S54, by the multiplier 502, and is output as transmission power-controlled amplitude data G·a(t). The amplitude data G·a(t) is amplified by the amplitude signal multiplier 503, and is supplied to the high frequency power amplifier 506 as a supply voltage.
The phase modulation section 504 modulates a carrier wave at an angular frequency ωc based on phase data exp[jφ(t)] to generate the high frequency phase-modulated signal S58. Where the high frequency phase-modulated signal S58 is Sc, Sc is represented by expression (2).Sc=exp[j(ωc×t+φ(t))]  (2)
The high frequency power amplifier 506 multiplies the amplitude data G·a(t) by the high frequency phase-modulated signal S59 which is input via the variable gain amplifier 505, and outputs the resultant signal as the high frequency transmission signal S60. Where the high frequency transmission signal S60 is high frequency signal Srf, the high frequency signal Srf is represented by expression (3).Srf=G·a(t)·exp[j(ωc×t+φ(t))]  (3)
The conventional transmission apparatus 500 using the polar modulation system can improve the power efficiency because the amplitude (envelope) of the signal which is output from the high frequency power amplifier 506 is controlled by the supply voltage and thus the high frequency power amplifier 506 can perform a saturated operation.
However, the conventional transmission apparatus 500 has the following problems.
The dynamic range of the transmission power varies in accordance with the wireless communication system. In the case where the dynamic range is wider on the lower side, namely, in the case of a wireless communication system providing a small minimum value of the transmission power, the amplitude signal needs to have a very low level. Accordingly, the conventional transmission apparatus 500 requires a very low level signal as the transmission power-controlled amplitude signal S55.
FIG. 12 shows different varying ranges of the amplitude signal S55 in accordance with the magnitude of the transmission power. In FIG. 12, the horizontal axis represents the amplitude signal S55 in antilogarithm in units of volts (V). The varying ranges of the amplitude signal S55 in accordance with the magnitude of the transmission power are each represented as a continuous signal by a two-head arrow line segment. The scale of the horizontal axis schematically shows that when the amplitude signal S55 is represented as a digital signal, the values thereof are made discrete (quantized). It is understood that when the signal amplitude S55 provided in an antilogarithm representation is handled as a digital signal, the ratio of the quantization noise to the varying range of the amplitude signal S55 is increased as the transmission power is decreased. Namely, as the transmission power is decreased, the representation precision of the amplitude signal S55 is decreased due to the quantization noise. This deteriorates an ACLR (Adjacent Channel Leakage power Ratio) representing a distortion of a transmission signal or an EVM (Error Vector Magnitude) representing a modulation precision of a transmission signal.
The influence on the ACLR and the EVM exerted by the quantization noise when the amplitude signal S55 provided in an antilogarithm representation is handled as a digital signal was obtained by simulation. FIG. 13 shows a result of the simulation regarding the ACLR, and FIG. 14 shows a result of the simulation regarding the EVM. For the simulation, an EDGE (Enhanced Data GSM Environment)-modulated signal was used.
FIG. 13 shows the ACLR obtained as a result of detuning at 400 kHz (see part (a)) and at 600 kHz (see part (b)). FIG. 14 shows the RMS EVM by the standards of 3GPP (3rd Generation Partnership Project). The simulation was performed with four quantization bit rates of 14 bits, 12 bits, 10 bits and 8 bits. The high frequency power amplifier 506 at the final stage, where polar modulation is performed in order to check the influence of the quantization noise, was assumed to have an ideal linear characteristic (with no distortion).
As can be understood from FIG. 13 and FIG. 14, when the amplitude signal S55 provided in an antilogarithm representation is handled as a digital signal, as the transmission power decreases, the ACLR and the EVM are deteriorated. At the same transmission power, the ACLR and the EVM are deteriorated at a larger degree as the quantization bit rate becomes smaller.
In the above description regarding the deterioration in the ACLR and the EVM caused by the quantization noise, the amplitude signal S55 is handled as a digital signal. In the case where the amplitude signal S55 is handled as an analog signal also, the S/N ratio (signal-to-noise ratio) is deteriorated where the amplitude signal S55 is of a very low level. Qualitatively, the ACLR and the EVM are deteriorated in a similar manner to the case of the digital signal.